Negative union: NMR on ions together

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  • Published: Oct 15, 2016
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Negative union: NMR on ions together

Not so repulsive

Proton nuclear magnetic resonance (NMR) spectroscopy has been used in research that reveals the first definitive evidence for a new type of molecular structure with a bond between two negatively charged moieties. Credit: Flood et al

Proton nuclear magnetic resonance (NMR) spectroscopy has been used in research that reveals the first definitive evidence for a new type of molecular structure with a bond between two negatively charged moieties.

The details, published in the journal Angewandte Chemie, shows how two bisulfate units can hook up and could have potential applications in improved nuclear waste storage materials as well as in chemical remediation of water contaminated withexcess fertiliser. Amar Flood of Indiana University, in Bloomington, explain that the existence of such a supramolecular structure was considered off-limits to solution- phase chemistry for the last two and a half centuries. The discovery opens up old chemical law to closer scrutiny and could have implications for a range of applications.

"An anion-anion dimerization of bisulfate goes against simple expectations of Coulomb's law," explains Flood, senior author on the Angewandte paper. "But the structural evidence we present in this paper shows two hydroxy anions can in fact be chemically bonded. We believe the long-range repulsions between these anions are offset by short-range attractions."

Breaking Coulomb's law

A key feature of Coulomb's law is the concept that two molecules with the same charge simply repel each other and so cannot form a bond. But, recent insights into the nature of chemical bonds from Frank Weinhold, at the University of Wisconsin, has begun to suggest that the presence of a hydrogen atom on a negatively charged moiety, such as a bisulfate, might veto this law and allow a union despite the charges.

"Although supramolecular chemistry started out as an effort to create new molecular hosts that hold on to complementary molecular guests through non-covalent bonds, the field has recently branched out to explore non-covalent interactions between the guests in order to create new 'chemical species,'" Elisabeth Fatila, a postdoctoral researcher in Flood's lab says. The negatively charged bisulfate dimer in the new study employs a self-complementary, anti-electrostatic hydrogen bond. This is facilitated by encapsulation inside a pair of cyanostar macrocycles invented previously in Flood's laboratory and wending their way through the patent application process. The team was trying to bind a single bisulfate molecule within the cyanostar unit but their spectroscopic and other data pointed to the presence of two negatively charged bisulfate ions within, which was surprising, to say the least.

Cleanup chemistry

The new chemical entity was detected using NMR in the chemistry department at Bloomington and in the Laboratory for Biological Mass Spectrometry. Co-authors on the paper are Krishnan Raghavachari, Jonathan Karty, NMR spectroscopist Eric Twum, Maren Pink, and Arkajyoti Sengupta.

With a negatively charged bisulfate dimer in hand it might now be possible to exploit this phenomenon to address seemingly intractable chemical problems such as the cleanup of waste water and lakes where excesses of ions, sulfates, phosphates and others cause ecological problems and algal blooms for instance. Ion extraction is often a key process in remediation and the new understanding of bisulfate bonding could be exploited in new approaches to cleanup. Similarly, the phenomenon might be used to efficiently extract sulfate ions from the nuclear waste storage materials and allow the vitrification products to contain more waste.

In August, Flood was named as principal investigator on a new project funded by the US National Science Foundation to focus on removing phosphates from the environment. The three-year, $600,000 award will support a collaboration with Heather Allen of The Ohio State University where large algal blooms due to agricultural runoff into Lake Erie have been problematic.

"The next step is to test the generality of the discovery," Flood told SpectroscopyNOW. "Does phosphate also form these dimers in solution inside the cyanostar receptors?" he asks. "Ultimately, we hope to understand anion recognition at a fundamental level such that we can create new receptors using computer-aided design and that we can use the recognition events to direct the synthesis of new materials using non-covalent bonding."

Related Links

Angew Chem 2016, online: "Anions Stabilize Each Other inside Macrocyclic Hosts"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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